1. Field of the Invention
The invention in general relates to the fabrication of integrated circuits, and more particularly to the fabrication of high dielectric constant and ferroelectric capacitors.
2. Statement of the Problem
Metal oxide materials, such as barium strontium titanate, commonly referred to as BST, are known to be useful in making integrated circuit thin film capacitors having high dielectric constants. See U.S. Pat. No. 5,122,923 issued to Shogo Matsubara et al., and Kuniaki Koyama, et al., xe2x80x9cA Stacked Capacitor With (BaxSr1xe2x88x92x)TiO3 For 256M DRAMxe2x80x9d in IDEM(International Electron Devices Meeting) Technical Digest, December 1991, pp. 32.1.1-32.1.4. Current integrated circuits, particularly DRAMs, are usually fabricated on substrates made of silicon and/or silicon dioxide and often may involve other silicon compounds. It has been known for many years that most metals do not adhere well to silicon, and thus adhesion layers of certain metals, such as titanium, that do adhere well to silicon have long been used as interfaces between metal layers and silicon. See S. M. Sze, VLSI Technology, second edition, McGraw-Hill Book Company, 1988, p. 382, and U.S. Pat. No. 5,046,043 issued to William D. Miller. It has also been known for many years that silicon diffuses into other material, particularly at raised temperatures, and thus diffusion barriers, such as titanium nitride (TiN), have long been used in integrated circuits to separated the silicon and other layers. See VLSI Technology, supra, p. 409. Therefore capacitors which have electrodes made of a metal, such as platinum, usually use a barrier layer, such as titanium nitride or titanium oxide, and an adhesion layer, such as titanium, between the capacitor electrode and a doped polysilicon, silicon dioxide, or silicon nitride layer. See U.S. Pat. Nos. 5,046,043, and 5,005,102 issued to William L. Larson. However, the foregoing patents disclose only the conventional processes for forming capacitors in combination with the adhesion and barrier layers. Such conventional processes, while useful in conventional silicon technology, when used in combination with capacitors that use metal oxides, such as barium strontium titanate (xe2x80x9cBSTxe2x80x9d), as the dielectric, result in capacitors that have relatively high leakage current, fatigue significantly, and generally have undesirable electrical properties. In particular, while the dielectric constant of bulk BST is of the order of 300 to 4000, the dielectric constant of thin films made according to the conventional processes is significantly lower. This is believed to be due to surface charges caused by defects and impurities in the films.
3. Solution to the Problem:
The invention solves the above problems by providing a process of making thin film capacitors in which the integrated circuit wafer is annealed after the deposition of the diffusion and barrier layers and prior to the formation of the metal electrode. In particular it has been found that annealing a titanium/titanium nitride adhesion/barrier layer prior to formation of a platinum electrode produces a BST capacitor of much lower leakage current than the prior art BST capacitors.
Further, it has been found that the Ti/TiN layers should preferably be annealed at temperatures higher than typical anneal temperatures in the prior art. In particular it had been found that an anneal temperature higher than 650xc2x0 C. and preferably of about 700xc2x0 C. should be used. Preferably the Ti/TiN annealing is performed in nitrogen.
The process according to the invention preferably includes deposition of a liquid precursor by a spin-on process. Preferably the liquid precursor is an alkoxycarboxylate precursor as described in U.S. patent application Ser. No. 08/132,744 filed Oct. 6, 1993, which is hereby incorporated by reference. The liquid precursor is dried and annealed to form the BST. Preferably the annealing is performed at a temperature between 700xc2x0 C. and 850xc2x0 C.
The invention also provides a capacitor structure in which the barrier layer is located between two layers of the same conducting material.
In an exemplary embodiment, capacitors made with a dielectric material comprising BST thin films having the formula Ba1xe2x88x92xSrxTiO3, where x is 0.03, were found to have a dielectric constant of nearly 500 and a leakage current of about 10xe2x88x929 amps/cm2 when made by the process of the invention. The dielectric constant is about twice as large and the leakage current is about ten times as small as the best respective properties reported in the prior art for BST thin films. Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.